1. Field of the Invention
The present invention relates to an electronic circuit, a modulation method, an information processing device, and an information processing method. More particularly, the present invention relates to an electronic circuit in which, when digital data is transmitted based on amplitude modulation via an antenna including a resonance circuit, the frequency of occurrence of communication failures at the receiving side is reduced, to a modulation method therefor, to an information processing device therefor, and to an information processing method therefor.
2. Description of the Related Art
An IC (Integrated Circuit) card system to which non-contact communication technology is applied is formed of a portable IC card and a commonly called reader/writer device (hereinafter referred to as an “R/W device”) that reads information stored in the IC card in a non-contact manner and that is capable of causing predetermined information to be stored in the IC card in a non-contact manner (for example, refer to Japanese Unexamined Patent Application Publication No. 10-13312 corresponding to U.S. Pat. No. 6,126,077).
More specifically, IC card systems are highly convenient systems capable of reading and writing information in a non-contact manner, and, for example, in recent years, have been used as alternative systems for conventional magnetic card systems typified by commuter passes and authentication cards.
Hitherto, the transmission section (the transmission section formed of an antenna and a section for transmitting digital data within a modulator-demodulator) within the R/W device of the IC card system is configured as shown in, for example, FIG. 1. In the following, such a transmission section will be referred to as a “transmission device” by assuming it to be a single device.
More specifically, as shown in FIG. 1, a transmission device 1 is provided with a carrier-wave output section 11 for outputting a carrier wave 21 of a predetermined frequency (for example, 13.56 MHz), a transmission data output section 12 for outputting a signal wave (rectangular wave) 22 of a predetermined frequency (for example, 212 kHz), which corresponds to the digital data to be transmitted (hereinafter referred to as “transmission data”), a modulation and amplification section 13 for generating an amplitude-modulated wave 23 by varying and amplifying the amplitude of the carrier wave 21 in accordance with the signal wave 22 and for outputting the amplitude-modulated wave 23, and an antenna section 14, having a resonance circuit formed of a coil La and a capacitor Ca, for outputting an electromagnetic wave 24 based on the amplitude-modulated wave 23 (for example, transmitting the electromagnetic wave 24 to an IC card (not shown)).
The modulation and amplification section 13 also functions as a drive section (drive) for the antenna section 14 when viewed from the antenna section 14, as will be described later. Accordingly, hereinafter, the modulation and amplification section 13 will also be referred to as an “antenna drive section 13”.
More specifically, for example, the transmission device 1 is configured as shown in FIG. 2.
That is, the modulation and amplification section 13 is formed of a transistor TR1 and a transistor TR2 for modulation and amplification, a switch SW, a resistor R1 and a resistor R2, which act as emitter loads of the transistor TR1 and the transistor TR2, a coil L1, a coil L2, a capacitor C1, and the primary side of a transformer Tr.
The carrier-wave output section 11 is connected to the bases of the transistor TR1 and the transistor TR2. However, a reverse phase of the carrier wave 21 input to the base of the transistor TR1 is input to the base of the transistor TR2.
One end of the resistor R1, whose other end is grounded, is connected to the emitters of the transistor TR1 and the transistor TR2, and one end of the resistor R2, whose other end is connected to the switch SW, is also connected thereto. The other end with regard to the end connected to the resistor R2 of the switch SW is grounded. This switch SW performs a switching operation in accordance with a change in a pulse signal 22 input from the transmission data output section 12.
One end of the coil L1, to whose other end a voltage Vcc1 is applied, is connected to the collector of the transistor TR1, and a resonance circuit formed of the capacitor C1 and the primary-side coil of the transformer Tr is also connected to the collector. In a similar manner, one end of the coil L2, to whose other end a voltage Vcc1 is applied, is connected to the collector of the transistor TR2, and a resonance circuit formed of the capacitor C1 and the primary-side coil of the transformer Tr is also connected to the collector.
The antenna section 14 is configured as a closed circuit in which a secondary-side coil La of the transformer Tr, a resistor Ra, and a capacitor Ca are connected in parallel. That is, the antenna section 14 operates in such a manner that the coil La part functions as a loop antenna, and also operates as an LCR resonance circuit formed of the coil La, the resistor Ra, and the capacitor Ca.
Next, the operation of the transmission device 1 of FIG. 2 will be described.
The carrier wave 21 of 13.56 MHz is always supplied from the carrier-wave output section 11 to the bases of the transistor TR1 and the transistor TR2.
In this state, when the pulse signal 22 of 212 KHz, corresponding to the transmission data, is output from the transmission data output section 12, the switch SW performs a switching operation in accordance with the change in the pulse signal 22. That is, when the switch SW is in an on state, since the resistor R2 is connected to the circuit, the emitter loads of the transistor TR1 and the transistor TR2 become the combined resistance value of the resistor R1 and the resistor R2. In contrast, when the switch SW is in an off state, since the resistor R2 is disconnected from the circuit, the emitter loads of the transistor TR1 and the transistor TR2 become the resistance value of the resistor R1.
In this manner, the emitter loads of the transistor TR1 and the transistor TR2 vary in accordance with the pulse signal 22 (transmission data) input from the transmission data output section 12. Then, as a result of the emitter loads of the transistor TR1 and the transistor TR2 being varied, the emitter current varies, and the amplitude of the collector voltage Vc also varies between two levels. That is, the collector voltage Vc corresponds to an amplitude-modulated wave 23.
The amplitude level of an electromagnetic wave 24 output from the coil La of the antenna section 14 also varies in accordance with the change of the amplitude of the collector voltage Vc (the amplitude-modulated wave 23).
In other words, the carrier wave 21 input to the bases of the transistor TR1 and the transistor TR2 is amplitude-modulated and amplified in accordance with the pulse signal 22 having two levels (by the change of the emitter current (signal wave) of the transistor TR1 and the transistor TR2, which corresponds to the change of the pulse signal 22), that is, a low level and a high level, forming the amplitude-modulated wave 23 having amplitudes of two levels, that is, a first level corresponding to the high level of the pulse signal 22 and a second level corresponding to the low level of the pulse signal 22. The amplitude-modulated wave 23 is supplied to the antenna section 14 via the collectors of the transistor TR1 and the transistor TR2. The antenna section 14 outputs the electromagnetic wave 24 based on the supplied amplitude-modulated wave 23, that is, the electromagnetic wave 24 having two amplitude levels corresponding to the first and second levels.
However, when the electromagnetic wave 24 (digital data corresponding to the pulse signal 22 superimposed onto the carrier wave 21) transmitted from the antenna section 14 of the transmission device 1 of FIG. 2 in accordance with the above-described operation is received by an IC card having a limiter for limiting the input and is demodulated, the data demodulated in the operating region of the limiter is distorted, and as a result, a problem arises in that communication failures occur often.